Evidence for a widespread involvement of NO in control of photogenesis in bioluminescent fish

The presence of nitric oxide synthase (NOS) and nerve fibres in the photophores of seven bioluminescent fish species (Hygophum benoiti, Myctophum punctatum, Electrona risso, Cyclothone braueri, Vinciguerria attenuata, Maurolicus muelleri and Porichthys notatus) with endogenous photocytes, were investigated. Antibodies directed against neuronal and inducible NOS (n and iNOS respectively) and NADPH-diaphorase activity were used to reveal the locations of NOS, while antibodies directed against acetylated tubulin were used to visualize nerve fibres. The nNOS antibody labelled structures in all investigated photophores except in the organs from P. notatus. The photocytes of P. notatus showed NADPH-diaph- orase activity. In the myctophid species, NOS-like immunoreactivity was found in small intracellular structures of the photocytes and in nerve fibres reaching the photocytes. nNOS-positive fibres were also found among lens ⁄ filter cells in V. attenuata, and in M. muelleri the cytoplasm of lens ⁄ filter cells contained NOS- like material. In C. braueri, a cell type located at a collecting chamber for lumi- nous products in the photophore contained NOS-like material. All photophores received an innervation reaching the photocytes, as well as other components including lens ⁄ filter areas. The results of this study comply with an involvement of nitric oxide in the control of bioluminescence in several fish species